This invention relates to an electrical bushing of a gas insulated electrical apparatus and more particularly to an electrical bushing of a gas insulated electrical apparatus comprising a high voltage central conductor and a grounded shield within an insulating tube.
FIG. 1 illustrates an example of a conventional electrical bushing which is disclosed in Japanese Patent Application Laid Open No. 56-30211.
The conventional electrical bushing comprises an insulating tube 1 such as a porcelain tube, an upper mounting fixture 1a secured to one end of the tube 1, an upper terminal 10 hermetically secured to the tube 1 through the fixture 1a, a lower mounting fixture 1b secured at the other end of the tube 1, a flange 4 which is grounded and to which the fixture 1b is hermetically secured and a vessel 5 provided at the top end thereof with a flange member 5a which is hermetically secured to the flange 4.
The electrical bushing further comprises a central conductor 2, which is used as a high voltage electrode and is axially disposed at the center of the hollow portion of the tube 1, a cylindrical shield 3, which is grounded and is secured at one end thereof to the inner circumferential edge of the flange 4 so as to surround the central conductor 2 and has an electric field moderating ring 3a formed at the other end thereof, and an insulating medium 6, such as SF.sub.6 gas, confined within the electrical apparatus.
Since it has been recently attempted to minimize the size of the insulating tube so that the production cost thereof may be cut down, the inner space of the tube 1 is very limited and accordingly the insulating distance between the field moderating ring 3a of the cylindrical shield 3 and the central conductor 2 becomes very short.
The thus constructed conventional electrical bushing has problems in that, when a high voltage is applied to the central conductor 2, the equipotential surfaces of the electric field are very dense between the field moderating ring 3a of the shield 3 and the central conductor 2 and the electric field strength around that portion is therefore considerably high, resulting in a limitation to the withstand voltage of the electrical bushing. Accordingly, the field moderating ring 3a must have a cross-sectional shape having as large a radius as possible and must be away from the central conductor 2 as far as possible. However, it is difficult to satisfy these conditions within the limited inner space of the tube 1.